A two-path bandpass Sigma Delta modulator for digital IF extraction at 20 MHz

Oversampled bandpass A/D converters based on sigma-delta (Sigma Delta) modulation can be used to robustly digitize the types of narrowband intermediate frequency (IF) signals that arise in radios and cellular systems, This paper proposes a two-path architecture for a fourth-order, bandpass modulator that is more tolerant of analog circuit limitations at high sampling speeds than conventional implementations based on the use of switched-capacitor biquadratic filters, An experimental prototype employing the two-path topology has been integrated in a 0.6-mu m, single-poly, triple-metal CMOS technology with capacitors synthesized from a stacked metal structure. Two interleaved paths clocked at 40 MHz digitize a 200-kHz bandwidth signal centered at 20 MHz with 75 dB of dynamic range while suppressing the undesired mirror image signal by 42 dB, At low input signal levels, the mixing of spurious tones at de and f(s)/2 with the input appears to degrade the performance of the modulator; out-of-band sinusoidal dither is shown to be an effective means of avoiding this degradation, The experimental modulator dissipates 72 mW from a 3.3 V supply.